In Optical Frequency Domain Reflectometry (OFDR), light from a tunable laser source is coupled into a measurement fiber, or more generally, a device under testing (DUT), and the reflected or backscattered light is made to interfere with light from the same source that has traveled along a reference path yielding information about the fiber, or the DUT.
For measurements on a fiber, in the case that the frequency of the laser source is swept linearly in time, the interference between the light that is coming from a single fixed scattering point on the measurement fiber and the reference light creates a detector signal that has a constant frequency, this frequency being proportional to the difference of the travel time of the light along the measurement path and the reference path. As the propagation velocity of the light and the length of the reference path are known, the position of the scattering point can be computed from the observed frequency.
When multiple scatterers are present in the measurement fiber, the detector signal will be a superposition of different frequencies, each frequency indicative of the position of the respective scatterer. A Fourier transform of the detector signal (a ‘scattering profile’) can be computed; in graphs of the amplitude and phase of the transformed signal, the amplitude and phase of the different frequencies that are present in the detector signal (which correspond to different scatterer positions) will be shown at their respective positions along the horizontal axis of the graph.
The amplitude and phase of the scattered light can be affected by external influences acting on the fiber. E.g., when the fiber is deformed by external stresses, or when the temperature of the fiber is modified, effects will be seen on the phase and/or amplitude of the scattering profile. From a comparison of the scattering profile of the fiber to the scattering profile of the same fiber in a reference state, information can be obtained about the external influences on the fiber as a function of position along the fiber; i.e. the fiber can be used for distributed sensing.
When stresses are applied to an optical fiber, e.g. when it is bent, birefringence is induced, which in general will cause a variation of the state of polarization of the light travelling along the fiber. The polarization state of light scattered at different positions of the fiber upon arrival at the detector will vary as well. Thus, light reflected from certain parts of the sensing fiber may have a polarization state at the detector, which is (nearly) orthogonal to the polarization state of the light that arrives at the detector via the reference path. Consequently, the strength of the interference signal coming from these certain parts of the sensing fiber will be very low. A known solution to this problem of ‘polarization fading’ is polarization-diverse detection, usually in the embodiment of a polarizing beam splitter (PBS) with separate detectors for the two polarization states transmitted by the PBS.
In a birefringent fiber, the refractive index depends on the state of polarization of the light. Consequently, the phases of the Fourier transforms of the detector signals in a polarization-diverse measurement will vary upon modification of the input polarization state of the light that is sent into the measurement fiber. In order to accurately assess the effect of an external influence on the fiber properties two measurements need to be performed; for the second measurement the input polarization state of the light sent to the fiber is made orthogonal to the polarization state used in the first measurement. In this manner, four detector signals are obtained (two detector signals for each of the two input polarization states). From the Fourier transforms of these four signals, a single effective scattering profile may be computed that, when compared to the effective scattering profile of the reference state, provides the desired information about the external influences on the fiber as a function of position. See, e.g. patent application US20110109898 A1. The length of the intermediate measurement time between the first measurement and the second measurement may, however, in some situations negatively affect the reliability of the result. Similarly, the dependency on two measurements rather than a single measurement can also reduce the effective rate at which the measurement process can be reliably repeated, e.g. if one measurement (i.e. a scan) is corrupt, due e.g. a wrong detector signal, the wavelength calibration and/or linearization could be incorrect or unusable, then the entire measurement process must be repeated.
WO 2007149230 discloses a polarization maintaining (PM) optical fiber having two polarization states being analyzed. First and second spectral responses of the PM fiber portion are determined. In a preferred implementation, the spectral responses are determined using Optical Frequency Domain Reflectometry (OFDR). Each polarization state of the PM fiber portion has a corresponding spectral component in the first spectral response. First and second spectral analyses of the PM fiber portion are performed using the first and second spectral responses. Based on those spectral analyses of the PM fiber portion, a first physical characteristic affecting the PM fiber portion is determined that is distinct from a second different physical characteristic affecting the fiber portion. An output signal related to the first physical characteristics affecting the fiber portion is provided, e.g., for display, further processing, etc.
The inventors of the present invention have appreciated that an improved Optical Frequency Domain Reflectometry (OFDR) system is of benefit, and have in consequence devised the present invention.